U.S. patent application number 13/051915 was filed with the patent office on 2012-03-29 for high surface area silicon carbide-coated carbon aerogel.
This patent application is currently assigned to Lawrence Livermore National Security, LLC. Invention is credited to Theodore F. Baumann, Joshua D. Kuntz, Joe H. Satcher, JR., Marcus A. WORSLEY.
Application Number | 20120077006 13/051915 |
Document ID | / |
Family ID | 45870948 |
Filed Date | 2012-03-29 |
United States Patent
Application |
20120077006 |
Kind Code |
A1 |
WORSLEY; Marcus A. ; et
al. |
March 29, 2012 |
HIGH SURFACE AREA SILICON CARBIDE-COATED CARBON AEROGEL
Abstract
A metal oxide-carbon composite includes a carbon aerogel with an
oxide overcoat. The metal oxide-carbon composite is made by
providing a carbon aerogel, immersing the carbon aerogel in a metal
oxide sol under a vacuum, raising the carbon aerogel with the metal
oxide sol to atmospheric pressure, curing the carbon aerogel with
the metal oxide sol at room temperature, and drying the carbon
aerogel with the metal oxide sol to produce the metal oxide-carbon
composite. The step of providing a carbon aerogel can provide an
activated carbon aerogel or provide a carbon aerogel with carbon
nanotubes that make the carbon aerogel mechanically robust. Carbon
aerogels can be coated with sol-gel silica and the silica can be
converted to silicon carbide, improving the thermal stability of
the carbon aerogel.
Inventors: |
WORSLEY; Marcus A.;
(Hayward, CA) ; Kuntz; Joshua D.; (Livermore,
CA) ; Baumann; Theodore F.; (Discover Bay, CA)
; Satcher, JR.; Joe H.; (Patterson, CA) |
Assignee: |
Lawrence Livermore National
Security, LLC
|
Family ID: |
45870948 |
Appl. No.: |
13/051915 |
Filed: |
March 18, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12694425 |
Jan 27, 2010 |
|
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13051915 |
|
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61315512 |
Mar 19, 2010 |
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Current U.S.
Class: |
428/219 ;
427/230; 427/379; 427/397.7; 428/307.7; 428/312.2; 977/742 |
Current CPC
Class: |
B01J 21/063 20130101;
B01J 27/22 20130101; B01J 37/084 20130101; C04B 35/532 20130101;
C04B 41/009 20130101; H01M 4/58 20130101; Y02E 60/10 20130101; B01J
21/185 20130101; B01J 23/06 20130101; C04B 2235/652 20130101; C04B
2235/77 20130101; H01M 4/48 20130101; Y10T 428/2918 20150115; C04B
41/4537 20130101; B82Y 30/00 20130101; C04B 2235/3856 20130101;
C04B 2235/80 20130101; H01G 11/32 20130101; B01J 21/06 20130101;
Y10T 428/2993 20150115; C04B 41/87 20130101; C04B 2235/3826
20130101; H01M 4/366 20130101; B01J 21/18 20130101; B01J 35/04
20130101; B01J 33/00 20130101; C04B 2235/46 20130101; Y10T
428/249967 20150401; B01J 23/70 20130101; B01J 27/224 20130101;
C04B 2235/441 20130101; H01G 11/36 20130101; H01G 11/46 20130101;
Y10T 428/2982 20150115; Y10T 428/249957 20150401; C04B 2235/616
20130101; C04B 35/83 20130101; B01J 23/34 20130101; C02F 1/4691
20130101; C04B 2235/5288 20130101; C04B 2235/606 20130101; C04B
35/624 20130101; C04B 35/62655 20130101; C04B 2235/3895 20130101;
Y02E 60/13 20130101; B01J 35/004 20130101; C04B 2235/3418 20130101;
C04B 2235/3232 20130101; C04B 2235/3886 20130101; C04B 41/009
20130101; C04B 35/52 20130101; C04B 38/00 20130101; C04B 41/4537
20130101; C04B 41/457 20130101; C04B 41/5041 20130101; C04B 41/4537
20130101; C04B 41/455 20130101; C04B 41/457 20130101; C04B 41/5061
20130101; C04B 41/5068 20130101; C04B 41/4537 20130101; C04B
41/4556 20130101; C04B 41/457 20130101; C04B 41/5035 20130101; C04B
41/5059 20130101 |
Class at
Publication: |
428/219 ;
428/312.2; 428/307.7; 427/397.7; 427/379; 427/230; 977/742 |
International
Class: |
B32B 5/18 20060101
B32B005/18; B05D 3/00 20060101 B05D003/00; B05D 7/22 20060101
B05D007/22; B32B 9/04 20060101 B32B009/04; B05D 3/02 20060101
B05D003/02 |
Goverment Interests
STATEMENT OF GOVERNMENT RIGHTS
[0002] The United States Government has rights in this invention
pursuant to Contract No. DE-AC52-07NA27344 between the United
States Department of Energy and Lawrence Livermore National
Security, LLC for the operation of Lawrence Livermore National
Laboratory.
Claims
1. A composition comprising at least one carbon aerogel and at
least one silica coating or at least one silicon carbide coating
disposed on the aerogel.
2. The composition of claim 1, wherein the composition comprises at
least one silicon carbide coating on the aerogel.
3. The composition of claim 1, wherein the composition comprises at
least one silicon carbide coating on the aerogel, and wherein the
oxygen content is zero according to energy dispersion x-ray
analysis.
4. The composition of claim 1, wherein the composition comprises at
least one silicon carbide coating on the aerogel, wherein the
coating comprises silicon carbide nanocrystals.
5. The composition of claim 1, wherein the composition comprises at
least one silica coating on the aerogel.
6. The composition of claim 1, wherein the composition has a
surface area of at least 2,000 m.sup.2/g.
7. The composition of claim 1, wherein the carbon aerogel is an
activated carbon aerogel.
8. The composition of claim 1, wherein the carbon aerogel comprises
carbon nanotubes.
9. The composition of claim 1, wherein the carbon aerogel has a
surface area of greater than 3,000 m.sup.2/g before the coating is
disposed on the aerogel.
10. The composition of claim 1, wherein the coating is a conformal
coating on inner surfaces of the aerogel.
11. The composition of claim 1, wherein the coating improves the
thermal stability of the carbon aerogel according to TGA.
12. The composition of claim 1, wherein the composition is mostly
microporous with remaining pore volume primarily in the large meso-
and macroporous range.
13. The composition of claim 1, wherein the silica comprises
sol-gel-derived silica.
14. The composition of claim 1, wherein the silicon carbide is
formed as a carbothermal reduction of a silica coating.
15. The composition of claim 1, wherein the coating comprises
substantially no silicon oxide.
16. The composition of claim 1, wherein the aerogel has a bimodal
porosity.
17. The composition of claim 1, wherein the carbon aerogel is an
activated carbon aerogel and the silicon carbide coating is
present, and the composition has a surface area of at least 2,000
m.sup.2/g.
18. The composition of claim 1, wherein the carbon aerogel is an
activated carbon aerogel and the silicon carbide coating is
present, and the composition has a surface area of at least 2,000
m.sup.2/g, and wherein the carbon aerogel has a surface area of
greater than 3,000 m.sup.2/g before the coating is disposed on the
aerogel.
19. A composition comprising at least one activated carbon aerogel
and at least one silicon carbide coating disposed on the activated
carbon aerogel, wherein the composition has a surface area of at
least 2,000 m.sup.2/g.
20. The composition of claim 19, wherein the silicon carbide
coating increases the thermal stability of the composition.
21. A method comprising: forming a silica coating on a carbon
aerogel, heating the coated aerogel to thermally convert the silica
coating into a silicon carbide coating.
22. The method of claim 21, wherein the silica comprises
sol-gel-derived silica.
23. The method of claim 21, further comprising drying the coated
aerogel by supercritical extraction in carbon dioxide prior to
heating.
24. The method of claim 21, wherein the silicon carbide coating
comprises a layer of SiC nanocrystals.
25. The method of claim 21, wherein the coated and heated aerogel
has a surface area greater than about 2,000 m.sup.2/g.
26. The method of claim 21, wherein the coating of silicon oxide
and silicon carbide are formed on inner surfaces of the
aerogel.
27. The method of claim 21, wherein the carbon aerogel is an
activated carbon aerogel.
28. The method of claim 21, wherein the carbon aerogel has a
bimodal porosity.
29. The method of claim 21, wherein the carbon aerogel has a
bimodal porosity, wherein one mode has a pore size of about 10 nm
or less, and the other mode has a pore size of about 100 nm or
more.
30. The method of claim 21, wherein carbon aerogel has a surface
area of at least about 3,000 m.sup.2/g.
31. A method, comprising: coating the inner surface of an activated
carbon aerogel (ACA) with a silica sol, reacting the silica sol to
undergo gelation and form a silica sol-gel, curing and drying the
sol-gel to nucleate silicon oxide particles to yield a
SiO.sub.2/ACA composite, and heating the SiO.sub.2/ACA composite to
form a SiC/ACA composite.
32. The method of claim 31, wherein the silica sol fills the pore
volume prior to gelation.
33. The method of claim 31, wherein the SiC/ACA composite comprises
a surface area greater than about 2,000 m.sup.2/g.
34. The method of claim 31, wherein during the heating step, the
SiO.sub.2 is completely converted to SiC.
35. The method of claim 31, wherein said heating comprises heating
at about 1,500.degree. C.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of application
Ser. No. 12/694,425 filed on Jan. 27, 2010, the disclosure of which
is hereby incorporated by reference herein in its entirety. This
application also claims the benefit of the filing date of U.S.
application No. 61/315512, filed on Mar. 19, 2010, the disclosure
of which is hereby incorporated by reference herein in its
entirety.
BACKGROUND
[0003] Porous metal oxides can be prepared by a number of
techniques ranging from sol-gel synthesis to various
templating/support methods. These porous metal oxides have shown
enhanced catalytic activity, compared to bulk material, but are
still limited by surface areas less than 1000 m.sup.2/g. This is
even the case when using high surface area templates such as SBA-15
or MCM-41. Surface areas for the templated metal oxides can be less
than 200 m.sup.2/g. The use of supports, such as carbon nanotubes,
also yields surface areas less than 300 m.sup.2/g. Another issue
presented by many porous metal oxides is that their pore structure
collapses at elevated temperatures. For example in titania
aerogels, this lack of pore stability results in order of magnitude
decreases in surface area under heating. The presence of silica has
been shown to provide some stabilization of pores at high
temperatures in titania-silica composites. However, the surface
area is still significantly decreased under heating.
[0004] Carbon nanotubes (CNTs) possess a number of intrinsic
properties that have made them promising materials in the design of
composite materials. CNTs can have electrical conductivities as
high as 10.sup.6 Sm.sup.-1, thermal conductivities as high as 3000
Wm.sup.-1K.sup.-1, elastic moduli on the order of 1 TPa, and are
extremely flexible. Unfortunately, the realization of these
properties in macroscopic forms such as foams and composites has
been limited. Foams, though conductive, tend to be mechanically
weak due to their dependence on van der Waals forces for mechanical
integrity.
[0005] The treatise, Introduction to Nanotechnology, by Charles P.
Poole, Jr., and Frank J. Owens. John Wiley &. Sons, 2003,
states: "Nanotechnology is based on the recognition that particles
less than the size of 100 nanometers (a nanometer is a billionth of
a meter) impart to nanostructures built from them new properties
and behavior. This happens because particles which are smaller than
the characteristic lengths associated with particular phenomena
often display new chemistry and physics, leading to new behavior
which depends on the size. So, for example, the electronic
structure, conductivity, reactivity, melting temperature, and
mechanical properties have all been observed to change when
particles become smaller than a critical size."
[0006] In addition, developing novel porous carbons and carbon
composites remains important for a range of current and emerging
technologies such as batteries, hydrogen storage, catalysis, and
adsorbents. (References 1-8) Porous carbons are promising
candidates for these applications because they possess high surface
areas, are chemically stable, and have high electrical
conductivities. Unfortunately, carbon has some key drawbacks that
limit its performance in certain cases. Carbon has a fairly low
resistance to oxidation at elevated temperatures, limiting the
operating temperature of carbon-supported catalysis in an oxidative
environment. A common way to improve the thermal stability of a
porous carbon is to cover its inner surface with a more thermally
stable material (e.g. an oxide or carbide) to serve as a barrier to
oxygen diffusion. (References 5,9) Typically, thermal stability is
improved but surface area is dramatically reduced. The reduction in
surface area occurs because the micropores, present in large
quantities in most porous carbons, are blocked by the depositing
species, decreasing the accessible active sites. Furthermore, if
high temperature treatment (e.g. carbothermal reduction) is used to
produce a carbide coating, additional surface area is lost due to
sintering. Therefore, though the thermal stability may be enhanced,
the surface area can be reduced to less than half that of the
original porous carbon. The design of a high surface area carbon
containing hierarchical porosity (micro- and macropores) could
minimize the instance of micropore blockage, providing a support
that could accept deposition of a thermally stable oxide or carbide
while maintaining a high surface area.
SUMMARY
[0007] Features and advantages of the present invention will become
apparent from the following description. Applicants are providing
this description, which includes drawings and examples of specific
embodiments, to give a broad representation of the invention.
Various changes and modifications within the spirit and scope of
the invention will become apparent to those skilled in the art from
this description and by practice of the invention. The scope of the
invention is not intended to be limited to the particular forms
disclosed and the invention covers all modifications, equivalents,
and alternatives falling within the spirit and scope of the
invention as defined by the claims.
[0008] The present invention in its various embodiments relates to
metal oxide and more particularly to a high surface area,
electrically conductive nanocarbon-supported metal oxide.
Additional embodiments also relate to nano-carbon supported silicon
carbide.
[0009] Embodiments described herein include compositions and
devices, methods of making compositions and devices, and methods of
using compositions and devices.
[0010] For example, the present invention in its various
embodiments provides a metal oxide-carbon aerogel composite that
includes a carbon aerogel with a metal oxide overcoat. The metal
oxide-carbon composite is made, in one embodiment, by providing a
carbon aerogel, immersing the carbon aerogel in a metal oxide sol
under a vacuum, returning the carbon aerogel with the metal oxide
sol to atmospheric pressure, curing the carbon aerogel with the
metal oxide sol at room temperature, and drying the carbon aerogel
with the metal oxide sol to produce the metal oxide-carbon
composite. The step of providing a carbon aerogel can be providing
an activated carbon aerogel or providing a carbon aerogel with
carbon nanotubes that make the carbon aerogel mechanically
robust.
[0011] Another embodiment provides, for example, a composition
comprising at least one carbon aerogel and at least one silica
coating or at least one silicon carbide coating disposed on the
aerogel. In one embodiment, the composition comprises at least one
silicon carbide coating on the aerogel. In one embodiment, the
composition comprises at least one silicon carbide coating on the
aerogel, and wherein the oxygen content is zero according to energy
dispersion x-ray analysis. In one embodiment, the composition
comprises at least one silicon carbide coating on the aerogel,
wherein the coating comprises silicon carbide nanocrystals. In one
embodiment, the composition comprises at least one silica coating
on the aerogel. In one embodiment, the composition has a surface
area of at least 2,000 m.sup.2/g. In one embodiment, the carbon
aerogel is an activated carbon aerogel. In one embodiment, the
carbon aerogel comprises carbon nanotubes. In one embodiment, the
carbon aerogel has a surface area of greater than 3,000 m.sup.2/g
before the coating is disposed on the aerogel. In one embodiment,
the coating is a conformal coating on the inner surfaces of the
aerogel. In one embodiment, the coating improves the thermal
stability of the carbon aerogel according to TGA. In one
embodiment, the composition is mostly microporous with remaining
pore volume primarily in the large meso- and macroporous range. In
one embodiment, the silica comprises sol-gel-derived silica. In one
embodiment, the silicon carbide is formed as a carbothermal
reduction of a silica coating. In one embodiment, the coating
comprises substantially no silicon oxide. In one embodiment, the
aerogel has a bimodal porosity. In one embodiment, the carbon
aerogel is an activated carbon aerogel and the silicon carbide
coating is present, and the composition has a surface area of at
least 2,000 m.sup.2/g. In one embodiment, the carbon aerogel is an
activated carbon aerogel and the silicon carbide coating is
present, and the composition has a surface area of at least 2,000
m.sup.2/g, and wherein the carbon aerogel has a surface area of
greater than 3,000 m.sup.2/g before the coating is disposed on the
aerogel.
[0012] Another embodiment provides a composition comprising at
least one activated carbon aerogel and at least one silicon carbide
coating disposed on the activated carbon aerogel, wherein the
composition has a surface area of at least 2,000 m.sup.2/g. In one
embodiment, the silicon carbide coating increases the thermal
stability of the composition.
[0013] Another embodiment provides, for example, a method
comprising: forming a silica coating on a carbon aerogel, heating
the coated aerogel to thermally convert the silica coating into a
silicon carbide coating. In one embodiment, the silica comprises
sol-gel-derived silica. Another embodiment further comprises drying
the coated aerogel by supercritical extraction in carbon dioxide
prior to heating. In another embodiment, the silicon carbide
coating comprises a layer of SiC nanocrystals. In another
embodiment, the coated and heated aerogel has a surface area
greater than about 2,000 m.sup.2/g. In another embodiment, the
coating of silica and silicon carbide are formed on inner surfaces
of the aerogel. In another embodiment, the carbon aerogel is an
activated carbon aerogel. In another embodiment, the carbon aerogel
has a bimodal porosity. In another embodiment, the carbon aerogel
has a bimodal porosity, wherein one mode has a pore size of about
10 nm or less, and the other mode has a pore size of about 100 nm
or more. In another embodiment, the carbon aerogel has a surface
area of at least about 3,000 m.sup.2/g.
[0014] Another embodiment provides, for example, a method,
comprising: coating the inner surface of an activated carbon
aerogel (ACA) with a silica sol, reacting the silica sol to undergo
gelation and form a silica sol-gel, curing and drying the sol-gel
to nucleate silicon oxide particles to yield a SiO.sub.2/ACA
composite, and heating the SiO.sub.2/ACA composite to form a
SiC/ACA composite. In another embodiment, the silica sol fills the
pore volume prior to gelation. In one embodiment, the SiC/ACA
composite comprises a surface area greater than about 2,000
m2/g.
[0015] In one embodiment, during the heating step, the SiO.sub.2 is
completely converted to SiC. In one embodiment, the heating
comprises heating at about 1,500.degree. C.
[0016] An advantage for at least one embodiment is high thermal
stability and/or high surface area for the aerogel, including the
combination of these properties.
[0017] The invention in its various embodiments has use as, for
example, a commercial catalyst. The invention in its various
embodiments also has use as an electrode, for example as an
electrode for batteries and super capacitors. The invention in its
various embodiments also has use in, for example, water
purification, electrical/electrochemical energy storage, solar
energy, and hydrogen storage.
[0018] The invention is susceptible to modifications and
alternative forms. Specific embodiments are shown by way of
example. It is to be understood that the invention is not limited
to the particular forms disclosed. The invention covers all
modifications, equivalents, and alternatives falling within the
spirit and scope of the invention as defined by the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIGS. 1A and 1B are SEM and TEM images of
TiO.sub.2/SWNT-CA.
[0020] FIG. 2 is a TGA plot of SWNT-CA, TiO.sub.2/SWNT-CA, and
TiO.sub.2 in air.
[0021] FIG. 3 is Semi-log plot of the pore size distribution of the
SWNT-CA, TiO.sub.2/SWNT-CA, and TiO.sub.2 aerogel.
[0022] FIGS. 4A-D are SEM images of TiO.sub.2/CNT (a, b) and
TiCN/CNT (c, d) at different magnifications.
[0023] FIGS. 5A and 5B are TEM images of TiO.sub.2/CNT and (b)
TiCNT/CNT.
[0024] FIGS. 6A-H are SEM images of ACA (a,b), as-prepared
TiO.sub.2/ACA (c,d), heat-treated TiO.sub.2/ACA (e,f), and TiCN/ACA
(g,h) at different magnifications. Arrows indicate particles of
amorphous (d), crystalline TiO.sub.2 (f), and TiCN (h).
[0025] FIGS. 7A-C are transmission electron microscopy images of
as-prepared TiO.sub.2/ACA (a), heat-treated TiO.sub.2/ACA (b), and
TiCN/ACA (c).
[0026] FIGS. 8A-D are SEM images of as-prepared SiO.sub.2/ACA and
SiC/ACA. The arrows indicate particles of SiO.sub.2. FIGS. 8a and
8b show low and high magnification SiO.sub.2/ACA, respectively.
[0027] FIGS. 8c and 8d show low and high magnification SiC/ACA,
respectively.
[0028] FIG. 9 is a flow chart showing one embodiment of a method of
making a metal oxide-carbon composite with carbon nanotubes that
make said metal oxide-carbon composite mechanically robust.
[0029] FIG. 10 is a flow chart showing one embodiment of a method
of making an metal oxide-carbon composite with an activated carbon
aerogel.
[0030] FIG. 11 is a plot of Si, O and C atomic content (EDX) during
carbothermal reduction of SiO.sup.2/ACA at 1500.degree. C. in Ar as
a function of time.
[0031] FIG. 12 shows powder XRD patterns for SiC/ACA and
SiO.sub.2/ACA.
[0032] FIG. 13 shows thermogravimetric analysis (TGA) plots of ACA,
SiO.sub.2/ACA, and SiC/ACA in air.
[0033] FIG. 14 shows nitrogen adsorption/desorption isotherms for
ACA, SiO.sub.2/ACA, and SiC/ACA.
DETAILED DESCRIPTION
Introduction
[0034] Referring to the drawings, to the following detailed
description, and to incorporated materials, detailed information
about the invention is provided including the description of
specific embodiments. The detailed description serves to explain
the principles of the invention. The invention is susceptible to
modifications and alternative forms. The invention is not limited
to the particular forms disclosed. The invention covers all
modifications, equivalents, and alternatives falling within the
spirit and scope of the invention as defined by the claims.
[0035] Embodiments described herein provide a metal oxide-carbon
composite that includes a carbon aerogel with an oxide overcoat.
The metal oxide-carbon composite is made by providing a carbon
aerogel, immersing the carbon aerogel in a metal oxide sol under a
vacuum, returning the carbon aerogel with the metal oxide sol to
atmospheric pressure, curing the carbon aerogel with the metal
oxide sol-gel at room temperature to produce the metal oxide-carbon
wet gel composite, and drying the metal oxide-carbon wet gel
composite to produce the metal oxide-carbon aerogel composite. The
step of providing a carbon aerogel can be providing an activated
carbon aerogel or providing a carbon aerogel with carbon nanotubes
that make the carbon aerogel mechanically robust. Apparatus and
method of providing an aerogel and a metal oxide are described in
U.S. Pat. No. 6,986,818, U.S. Pat. No. 7,270,851; U.S. Pat. No.
7,410,718; U.S. Published Patent Application No. 20090123358;
Published Patent Application No. 20090229032; and Published Patent
Application No. 20090317619. U.S. Pat. No. 6,986,818, U.s. Pat. No.
7,270,851; U.S. Pat. No. 7,410,718; U.S. Published Patent
Application No. 20090123358; Published Patent Application No.
20090229032; and Published Patent Application No. 20090317619 are
incorporated herein in their entirety by this reference for all
purposes.
Definition Terms
[0036] Various terms used in this patent application are defined
below.
[0037] CA=Carbon Aerogel
[0038] CNT=Carbon Nanotubes
[0039] CA-CNT=Carbon Aerogel & Carbon Nanotube Composite
[0040] SWNT=Single-Walled Carbon Nanotubes
[0041] DWNT=Double-Walled Carbon Nanotubes
[0042] SDBS=Sodium Dodecylbenzene Sulfonate
[0043] MESOPOROUS=Pore Dia. 2-50 nm
[0044] PVA=Polyvinyl Alcohol
[0045] CVD=Chemical Vapor Deposition
[0046] TEM=Transmission Electron Microscopy
[0047] SEM=Scanning Electron Microscopy
[0048] R/C=Resorcinol to Catalyst Ratios
[0049] RF=Resorcinol and Formaldehyde Solids
[0050] BET=Brunauer-Emmett-Teller
[0051] Mechanically Robust=Can withstand strains greater than 10%
before fracture
[0052] Electrically Conductive=Exhibits an electrical conductivity
of 10 S/m or greater
[0053] Ultralow-Density=Exhibits densities less than 50 mg/cc
[0054] Carbon Nanotube-Based Aerogel=Porous carbon material
consisting of 5 to 95% carbon nanotubes by weight.
EXAMPLES
[0055] The present invention provides a method of making a metal
oxide-carbon composite, comprising the steps of providing an
aqueous media or other media to form a suspension, adding reactants
and catalyst to said suspension to create a reaction mixture,
curing said reaction mixture to form a wet gel, drying said wet gel
to produce a dry gel, pyrolyzing said dry gel to produce an
aerogel,immerse said aerogel in a metal oxide sol under a vacuum,
returning said aerogel and said metal oxide sol to atmospheric
pressure, curing said aerogel at room temperature, and drying said
aerogel producing an aerogel oxide composite. The metal
oxide-carbon composite comprises a carbon aerogel, said carbon
aerogel having inner surfaces, and an oxide coating said inner
surfaces of said carbon aerogel providing an aerogel oxide
composite. In one embodiment the carbon aerogel is a carbon aerogel
with carbon nanotubes that make said carbon aerogel mechanically
robust. In another embodiment the carbon aerogel is an activated
carbon aerogel. In one embodiment the oxide is titanium oxide. In
another embodiment the oxide is an oxide from transitional metal
oxide made with forming precursors of manganese or iron or cobalt
or nickel or copper or zinc or zirconium or tin salts or
alkoxides.
Nanocarbon-Supported Titanium Dioxide
[0056] The present invention in its various embodiments provides
the fabrication of new nanocarbon supported titanium dioxide
structures that exhibit high surface area and improved electrical
conductivity. Nanocarbons consisting of single-walled carbon
nanotubes and carbon aerogel nanoparticles were used to support
titanium dioxide particles and produce monoliths with densities as
low as 80 mg/cm.sup.3. The electrical conductivity of the
nanocarbon-supported titanium dioxide was dictated by the
conductivity of the nanocarbon support while the pore structure was
dominated by the titanium dioxide aerogel particles. The
conductivity of the monoliths presented here was 0.72 S/cm and the
surface area was 203 m.sup.2/g.
[0057] Titanium dioxide is a widely researched material with
applications ranging from photocatalysts to electrodes to hydrogen
storage materials. However, issues such as absorption limited to
the ultraviolet range, high rates of electron-hole recombination,
and relatively low surface areas have limited commercial use of
titanium dioxide. Recent efforts have focused on combining titanium
dioxide with various materials to address some of these issues.
Titanium dioxide in the presence of carbon (e.g. carbon nanotubes
(CNT)) is currently one of the most attractive combinations. While
recent work has shown some improvements, surfaces areas and
photocatalytic activity are still limited. Maintaining high surface
areas while improving electrical conductivities, one could envision
charging-discharging rates and photoefficiencies that are
significantly higher than currently possible. Unfortunately for CNT
composites, improvements in electrical conductivity are often not
fully realized due to poor dispersion of CNTs in the matrix
material, impeding the formation of a conductive network. However,
with a mechanically robust, electrically conductive CNT foam, one
could imagine simply coating this low-density CNT scaffold with
titanium dioxide, yielding conductive nanocarbon-supported titanium
dioxide.
[0058] Here, Applicants present the synthesis and characterization
of such a high-surface area, conductive TiO.sub.2/CNT composite.
Applicants recently reported the synthesis of a novel CNT-based
foam, consisting of bundles of single-walled nanotubes (SWNT)
crosslinked by carbon aerogel (CA) nanoparticles, which would serve
as an excellent candidate for the CNT scaffold of the TiO.sub.2/CNT
composite. The SWNT-CA foams simultaneously exhibited increased
stiffness, and high electrical conductivity even at densities
approaching 10 mg cm.sup.3 without reinforcement. The foams are
stable to temperatures approaching 1000.degree. C. and have been
shown to be unaltered by exposure to extremely low temperatures
during immersion in cryogenic liquids. So, in addition to their use
in applications such as catalyst supports, sensors, and electrodes,
these ultralight, robust foams could allow the formation of novel
CNT composites. As the conductive network is already established,
it can be impregnated through the wicking process with a matrix of
choice, ranging from inorganic sols to polymer melts to ceramic
pastes. Thus, a variety of conductive CNT composites could be
created using the SWNT-CA foam as a pre-made CNT scaffold.
Applicants use the SWNT-CA as a scaffold for the synthesis of
conductive, high surface area TiO.sub.2/CNT composites.
Experiment
Materials
[0059] All reagents were used without further purification.
Resorcinol (99%) and formaldehyde (37% in water) were purchased
from Aldrich Chemical Co. Sodium carbonate (anhydrous) was
purchased from J. T. Baker Chemical Co. Highly purified SWNTs were
purchased from Carbon Solutions, Inc.
[0060] SWNT-CA preparation. The SWNT-CAs were prepared as described
in previous work. Briefly, in a typical reaction, purified SWNTs
(Carbon Solutions, Inc.) were suspended in deionized water and
thoroughly dispersed using a VWR Scientific Model 75T Aquasonic
(sonic power - 90 W, frequency -40 kHz). The concentration of SWNTs
in the reaction mixture was 0.7 wt %. Once the SWNTs were
dispersed, resorcinol (1.235 g, 11.2 mmol), formaldehyde (1.791 g,
22.1 mmol) and sodium carbonate catalyst (5.95 mg, 0.056 mmol) were
added to the reaction solution. The resorcinol to catalyst ratios
(R/C) employed was 200. The amount of resorcinol and formaldehyde
(RF solids) used was 4 wt %. The sol-gel mixture was then
transferred to glass molds, sealed and cured in an oven at
85.degree. C. for 72 h. The resulting gels were then removed from
the molds and washed with acetone for 72 h to remove all the water
from the pores of the gel network. The wet gels were subsequently
dried with supercritical CO.sup.2 and pyrolyzed at 1050.degree. C.
under a N2 atmosphere for 3 h. The SWNT-CAs materials were isolated
as black cylindrical monoliths. Foams with SWNT loadings of 30 wt %
(0.5 vol %) were prepared by this method.
TiO.sub.2/SWNT-CA composite preparation
[0061] Sol-gel chemistry was used to deposit the TiO, aerogel layer
on the inner surfaces of the SWNT-CA support. The TiO.sub.2 sol-gel
solution was prepared as described in previous work. In a typical
synthesis, SWNT-CA parts were immersed in the TiO.sub.2 sol-gel
solution and full infiltration of the SWNT-CA pore network by the
sol-gel solution was achieved under vacuum. Following gelation of
the titania network, the wet composite was dried using
supercritical CO.sub.2, yielding the TiO.sub.2/SWNT-CA
composite.
TiO.sub.2/SWNT-CA Characterization
[0062] Bulk densities of the TiO.sub.2/SWNT-CA composites were
determined from the physical dimensions and mass of each sample.
The volume percent of SWNT in each sample was calculated from the
initial mass of SWNTs added, assuming a CNT density of 1.3
g/cm.sup.3, and the final volume of the aerogel. Scanning electron
microscopy (SEM) characterization was performed on a JEOL 7401-F at
10 keV (20mA) in SEI mode with a working distance of 2 mm.
Transmission electron microscopy (TEM) characterization was
performed on a JEOL JEM-200CX. Thermogravimetric analysis (TGA) was
performed on a Shimadzu TGA 50 Thermogravimetric Analyzer to
determine TiO.sub.2 content. Samples were heated in flowing air at
10 sccm to 1000.degree. C. at 10.degree. C./min in alumina boats.
The weight fraction of material remaining was assumed to be pure
stoichiometric TiO.sub.2. Energy dispersive spectroscopy confirmed
that only TiO, remained after TGA was performed. Surface area
determination and pore volume and size analysis were performed by
Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH)
methods using an ASAP 2000 Surface Area Analyzer (Micromeritics
Instrument Corporation). Samples of approximately 0.1 g were heated
to 300.degree. C. under vacuum (10.sup.-5 Torr) for at least 24
hours to remove all adsorbed species. Electrical conductivity was
measured using the four-probe method similar to previous studies.
Metal electrodes were attached to the ends of the cylindrical
samples. The amount of current transmitted through the sample
during measurement was 100 mA and the voltage drop along the sample
was measured over distances of 3 to 6 mm. Seven or more
measurements were taken on each sample.
[0063] The microstructure of the TiO.sub.2/SWNT-CA composites was
examined using SEM and TEM. As shown in FIG. 1A and FIG. 1B, the
network structure of the TiO.sub.2,/SWNT-CA composites is similar
to that observed in pristine SWNT-CA. The presence of the TiO,
aerogel layer on the surface of the nanotube bundles can be seen in
TEM image. Interestingly, the TiO.sub.2, aerogel appears to have
formed primarily on the surfaces of the nanotube bundles despite
the fact that the TiO.sub.2 sol-gel solution filled the entire pore
volume of the support. The open pore volume in the
TiO.sub.2/SWNT-CA composite is only sparsely populated with
TiO.sub.2 particles. This observation indicates that nucleation of
the TiO.sub.2 particles during the sol-gel reaction preferentially
occurs at the surface of the nanotube bundles.
[0064] Thermal gravimetric analysis in air was used to determine
the TiO.sub.2 content in the as-TiO,/SWNT-CA composites as
illustrated in FIG. 2. As expected, combustion of the pristine
SWNT-CA occurs around 500.degree. C. and the material is completely
consumed by 600.degree. C. The 5 wt % remaining is likely metal
catalyst from the CNTs. The titania exhibits an initial mass loss
generally attributed to moisture and organics below 300.degree. C.
and is stable thereafter. Not surprisingly, the TGA plot for
TiO.sub.2/SWNT-CA material is a composite of the plots for titania
and the SWNT-CA. It is interesting to note that the combustion of
the SWNT-CA occurs significantly earlier for the TiO.sub.2/SWNT-CA
compared to that for the pristine SWNT-CA, which may be the result
of a catalytic effect of the titania aerogel particles on carbon
oxidation. Nevertheless, the nearly 50 wt % remaining after
combustion of the SWNT-CA confirm the presence of titania in the
TiO.sub.2/SWNT-CA composite.
[0065] FIG. 3 plots the pore size distribution of the SWNT-CA,
TiO.sub.2/SWNT-CA composite, and pristine TiO, aerogel. The BET
surface area, electrical conductivity and other physical properties
of these materials are summarized in Table 1. Table I shows that
the TiO.sub.2/SWNT-CA composite has high surface area and
electrical conductivity. In fact, the electrical conductivity of
the SWNT-CA is not adversely affected by the infiltration of the
insulating material. Though, based on the SEM and TEM images (FIG.
1), the titania aerogel appears to simply coat the SWNT-CA
scaffold, the increased surface area suggests that the pore
morphology of the titania dominates the overall pore morphology of
the composite. This is confirmed via the pore size distribution,
which shows that the pore size distribution of the
TiO.sub.2/SWNT-CA is much closer to that of pristine TiO.sub.2
aerogel than that of the SWNT-CA. Thus, with the TiO.sub.2/SWNT-CA
composite, a new class of materials with good electrical
conductivity and high surface area are realized.
TABLE-US-00001 TABLE 1 Physical Properties of SWNT-CA,
TiO.sub.2/SWNT-CA, and TiO.sub.2 aerogel. CNT, vol % Density,
S.sub.BET, .sigma., Material (wt %) g/cm.sup.3 m.sup.2/g S
cm.sup.-1 SWNT-CA 0.5 (30) 0.030 184 0.77 TiO.sub.2/SWNT-CA 0.5 (8)
0.082 203 0.72 TiO.sub.2 aerogel 0 (0) 0.193 237 <0.001
[0066] Applicants have described a straightforward method for the
fabrication of electrically conductive, high-surface area
TiO.sub.2/CNT composites. The novel TiO.sub.2/SWNT-CA monoliths was
prepared by coating the CNT struts within the SWNT-CA scaffold with
amorphous sol-gel-derived TiO.sub.2, particles. Given the
technological interest in crystalline TiO.sub.2, one can convert
the amorphous TiO.sub.2, layer to the anatase crystalline phase.
The conductive network of the SWNT-CA scaffold remained intact
after infiltration yielding a composite with a conductivity of 0.72
S-cm.sup.-1 and a surface area of 203 m.sup.2/g. Therefore, the
SWNT-CAs were shown to provide the means to create conductive,
high-surface area TiO.sub.2, composites. The general nature of this
method should provide a route for the synthesis of a variety of
conductive, high surface area composites with applications in
photocatalysis and energy storage.
[0067] This nanocarbon-supported titanium dioxide example is
described in greater detail in the journal article, "Synthesis and
Characterization of Nanocarbon-Supported Titanium Dioxide,"
Author(s): Marcus A Worsley, Joshua D. Kuntz, Octavio Cervantes, T
Yong-Jin Han, Peter Pauzauskie, Joe H. Satcher, Theodore F.
Baumann, Paper #: 1174-V03-06, DOI: 10.1557/PROC-1174-V03-06, 2010
MRS Spring Meeting, Material Research Society. The journal article
"Synthesis and Characterization of Nanocarbon-Supported Titanium
Dioxide," by Marcus A. Worsley, Joshua D. Kuntz, Octavio Cervantes,
T. Yong-Jin Han, Peter J. Pauzauskie, Joe H. Satcher, Jr. and
Theodore F. Baumann, Mater. Res. Soc. Proc. Vol. 1174, (2009) is
incorporated herein in its entirety by this reference for all
purposes.
Example
High Surface Area Carbon Nanotube-Supported Titanium Carbonitride
Aerogels
[0068] Porous transition metal nitrides and carbides have received
considerable attention recently as catalysts and catalyst supports.
They exhibit high resistance to sintering and poisoning, in
addition to catalytic activity for a number of useful reactions. Of
particular interest is the fact that these transition metal
compounds have been shown to have catalytic activity similar to
that of typical noble metal catalysts. Thus, substituting
transition metal compounds for noble metals is an attractive option
for reducing the cost of catalyst materials. Unfortunately,
traditional routes to forming metal nitrides and carbides, such as
the carbothermal reduction of metal oxides, yield low surface area
materials. To increase the specific surface area of transition
metal carbides and nitrides, a number of new synthetic methods have
been proposed. One promising approach involves the use of high
surface area templates or supports to control the microstructure of
the transition metal nitride and carbide. For example, both high
surface area SiO.sub.2 and C.sub.3N.sub.4 have been used to form
TiN powders with surface areas in excess of 100 m.sup.2/g. With
surface areas as high as 1000 m.sup.2/g, carbon nanotubes (CNT)
could also serve as such a high surface area support. There have
been a number of studies exploring the deposition of various metal
oxides on CNTs, however, to our knowledge, only one study examines
depositing a transition metal nitride on CNTs. And while the
fabrication of metal nitride or carbide nanostructures has received
a lot of attention, the use of CNTs for creating high surface area
transition metal nitrides or carbides has not been reported.
[0069] Here, Applicants report the synthesis and characterization
of a monolithic CNT-supported titanium carbonitride aerogel
(TiCN/CNT) with surface area in excess of 250 m.sup.2/g. This
TiCN/CNT was formed by the carbothermal reduction of a
TiO.sub.2-coated low-density CNT-based foam (TiO.sub.2/CNT) in
flowing nitrogen. The CNT-based foam (30 wt % CNT, 30 mg cm.sup.-3)
that serves as the support consists of single-walled carbon
nanotubes crosslinked by carbon aerogel particles (SWNT-CA), as
previously described. To prepare the TiO.sub.2,/CNT, the SWNT-CA
was immersed in a TiO, sol under vacuum prior to gelation, similar
to the method previously reported for fabricating stiff, conductive
polymer/CNT composites. The TiO.sub.2 sol was prepared via a
two-step sol-gel process involving the acid-catalyzed hydrolysis of
titanium tetraethoxide, followed by base-initiated gelation of the
TiO.sub.2 species. Briefly, a solution of titanium tetraethoxide
(1.0 g, 4.4 mmol) and pure ethanol (4.5 mL) was prepared in an ice
bath with vigorous stirring. Once chilled, hydrochloric acid (37%,
71.4 .mu.L) and deionized water (85.7 .mu.L) were then added to the
titanium tetraethoxide/ethanol solution. After five minutes of
continuous stirring, propylene oxide (0.36 g, 6.1 mmol) was finally
added to the reaction mixture. The reaction mixture was stirred for
another five minutes before immersing the SWNT-CA monolith in the
TiO.sub.2 sol. Vacuum was applied to the reaction vessel to ensure
complete infiltration of the TiO.sub.2 sol in the SWNT-CA. After
infiltration, the TiO.sub.2 sol was then allowed to gel in the
SWNT-CA under ambient conditions. The wet composite gel was then
dried using supercritical CO.sub.2, yielding the TiO.sub.2/CNT. The
TiO.sub.2/CNT was then heated under flowing nitrogen at
1400.degree. C. for 4 hours to yield the TiCN/CNT monolith.
[0070] Powder X-ray diffraction (XRD) analysis of the samples was
performed with Cu Ka radiation on a Scintag PAD-V X-ray
diffractometer. TiO.sub.2 powder was used as a standard. Bulk
densities of the monoliths were determined from the physical
dimensions and mass of each sample. Scanning electron microscopy
(SEM) and energy-dispersive X-ray spectroscopy (EDX)
characterization were performed on a JEOL 7401-F at 5-10 keV (20mA)
in SEI mode with a working distance of 2-8 mm. To supplement EDX,
thermogravimetric analysis (TGA) was performed on a Shimadzu TGA 50
Thermogravimetric Analyzer. Samples were heated in air to
1000.degree. C. at 10.degree. C./min in alumina boats. Transmission
electron microscopy (TEM) characterization was performed on a JEOL
JEM-200CX Electron Microscope operated at 200 kV. Samples for TEM
were prepared by pulverizing aerogels above TEM grids. Surface area
determination and pore volume and size analysis were performed by
Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH)
methods using an ASAP 2000 Surface Area Analyzer (Micromeritics
Instrument Corporation). Samples of approximately 0.1 g were heated
to 300.degree. C. under vacuum (10.sup.-5 Torr)for at least 24
hours to remove all adsorbed species prior to analysis. Electrical
conductivity was measured using the four-probe method similar to
previous studies. Metal electrodes were attached to the ends of
cylindrical samples. The amount of current transmitted through the
sample during measurement was 100 rnA, and the voltage drop along
the sample was measured over distances of 3 to 6 mm.
[0071] SEM images of the TiO.sub.2/CNT, FIG. 4A and FIG. 4B and
TiCN/CNT) FIG. 4C and FIG. 4D show the ligament and pore structure
of these materials. The TiO.sub.2/CNT resembles the CNT-based foam
except for the coating of amorphous TiO.sub.2. The TEM image of the
TiO.sub.2/CNT, FIG. 2A supports this view. The TiCN/CNT also has
the same basic structure as the original CNT-based foam except that
the ligaments are now decorated with TiCN nanocrystals FIGS. 4B and
4C. This observation suggests that the carbon consumed during the
reduction of TiO, comes primarily from the carbon aerogel coating
the CNT bundles, leaving the CNTs intact. The integrity of the CNTs
was also confirmed via Raman spectroscopy through observation of
the peaks characteristic of CNTs (ESIl) in the TiCN/CNT. The TEM
image, FIG. 2B, also shows that the TiCN/CNT ligaments, on average,
have smaller diameters than the TiO.sub.2/CNT. The smaller
diameters probably occur as the TiO, is reduced and carbon aerogel
is consumed in the course of forming the TiCN nanocrystals. The
TiCN/CNT had a brownish color compared to the jet-black CNT-based
foam and TiO.sub.2/CNT.
TABLE-US-00002 TABLE II Density (.rho.), electrical conductivity
(.sigma.), and elemental content (Ti, C, N, O) of the composite
foams .rho., .sigma., Ti, at % C, at % N, at % O, at % Material g
cm.sup.-3 S cm.sup.-1 (wt %) (wt %) (wt %) (wt %) CNT-based 0.030
0.77 -- 95 (93) -- 5.0 (6.6) foam TiO.sub.2/CNT 0.082 0.72 9.4 (28)
71 (53) -- 19 (19) TiCN/CNT 0.055 0.25 17 (43) 65 (43) 18 (14)
<1 (<1)
[0072] Table II summarizes some basic properties of the TiCN/CNT,
as well as the CNT-based foam and the TiO.sub.2/CNT. The density of
the TiCN/CNT is significantly reduced compared to the
TiO.sub.2/CNT. During the carbothermal reduction, the monolith
experienced 49% mass loss and 28% volume shrinkage, resulting in
the 55 mg cm.sup.-3 final density. The electrical conductivity of
the TiCN/CNT is diminished compared to the CNT-based foam and
TiO.sub.2/CNT, but still high considering the extremely low bulk
density of the TiCN/CNT foam. The partial consumption during the
heat treatment of the graphitic carbon aerogel particles that
crosslink the CNT bundles, is likely the cause of the decreased
conductivity. Interfacial resistance has been shown to be a
dominant factor in the transport properties of CNT composites. The
removal or narrowing of the critical conduction pathways between
CNT bundles effectively increases the interfacial resistance,
leading to a decrease in the bulk conductivity.
[0073] Elemental analysis by EDX and TGA suggests that the
TiO.sub.2, in the TiO.sub.2,/CNT is completely converted to TiCN in
the TiCN/CNT. This observation is consistent with literature on the
carbothermal reduction of TiO, under the conditions of this study.
Under a constant supply of nitrogen and excess carbon, it is
expected that 100% reduction should occur, assuming temperature and
time are chosen appropriately. Previous studies have shown 100%
reduction at temperatures as low as 1300 DC for a 4 hour hold time.
The roughly 1:1 Ti:N ratio suggests a fairly N-rich TiCN phase was
formed. EDX elemental mapping (ESI) shows an even distribution of
elements indicative of a TiCN layer that covers most of the CNT
surface. XRD analysis offers more details concerning the
composition of the TiCN phase.
[0074] Powder XRD was used to determine what phases were present in
the TiCN/CNT. For reference, XRD patterns of the CNT-based foam and
TiO.sub.2/CNT were also included. The largest peaks from the
CNT-based foam can be attributed to the (100) and (101) graphite
peaks (PDF #41-1487). These peaks are also visible in the pattern
from the TiO.sub.2/CNT. The absence of additional peaks in the
TiO.sub.2/CNT pattern supports the earlier suggestion that the
TiO.sub.2 coating the CNT ligaments is amorphous. The XRD peaks for
the TiCN/CNT would indicate the presence of the osbornite
crystalline phase of TiCN (PDF #06-642). The calculated lattice
parameter, a, for the TiCN/CNT, 4.244 A, is in good agreement with
TiC.sub.1-xN.sub.x (x=0.95) and very close to the value for pure
TiN, 4.240. Peak broadening indicates that the average crystallite
size is about 20 nm, consistent with the particle sizes observed in
SEM and TEM analysis and. Therefore, based on the XRD data, a
highly nitrogen-enriched layer of TiCN nanocrystals covers the CNT
bundles.
[0075] Nitrogen adsorption/desorption analysis was performed to
determine surface area, pore volume and average pore size of the
TiCN/CNT. All three samples had Type IV nitrogen isotherms (ESI),
indicative of the predominantly macroporous nature of the CNT-based
foam that serves as the foundation for all the samples. The
addition of TiO.sub.2 and the conversion to TiCN increased both the
surface area and pore volume of the composite foams. Peak pore size
increases from 56 nm in the CNT-based foam to 72 nm in the
TiO.sub.2/CNT and TiCN/CNT. The TiO.sub.2/CNT exhibits pore
morphology similar to that of an amorphous TiO.sub.2 aerogel,
suggesting that the TiO.sub.2 coating the CNT bundles dominates the
nitrogen sorption behavior. The TiCN/CNT maintains the same general
morphology as the TiO.sub.2/CNT, as evidenced by a similar pore
size distribution. However, the surface area and pore volume are
increased because of the decreased bulk density and additional
porosity due to removal of carbon (in the form of gaseous CO) that
occurs during carbothermal reduction. Similar increases in surface
area were observed by Berger et al. under similar conditions during
the conversion of TiO.sub.2 (rutile) and carbon (furnace black or
graphite).
[0076] In summary, the synthesis of high surface area TiCN/CNT has
been shown by the carbothermal reduction of TiO.sub.2 in a
CNT-based foam. The resulting monolith was conductive, contained
N-rich TiCN nanocrystals decorating CNT bundles and had a surface
area of 276 m.sup.2/g. The straightforward nature of this method
should allow for the synthesis of other high surface area
CNT-supported metal nitrides (e.g. ZrN, Si.sub.3N.sub.4) by simply
reducing the respective oxide (e.g. ZrO.sub.2, SiO.sub.2). Also, by
performing the carbothermal reduction in inert gas (e.g. Ar), high
surface area carbides (e.g. TiC, SiC) could also be formed. Thus, a
new class of monolithic, high surface area CNT-supported carbides
and nitrides could be developed with potential for significant
contributions in applications such as catalysis.
[0077] This high surface area carbon nanotube-supported titanium
carbonitride aerogels example is described in greater detail in the
journal article "High surface area carbon nanotube-supported
titanium carbonitride aerogels," by Marcus A. Worsley, Joshua D.
Kuntz, Peter J. Pauzauskie, Octavio Cervantes, Joseph M. Zaug, Alex
E. Gash, Joe H. Satcher Jr., and Theodore F. Baumann, Journal of
Materials Chemistry, 2009, 19, 5503-5506, which is incorporated
herein in its entirety by this reference for all purposes.
Example
High Surface Area TiO.sub.2/C and TiCN/C Composites
[0078] Nanocomposites of titania and various forms of carbon (i.e.
carbon nanotubes, activated carbons, ordered carbons, etc.) exhibit
a number of enhanced functional properties for catalysis and
energy-storage applications. Several reports have shown that
titania/carbon (TiO.sub.2/C) composites have higher photocatalytic
activity, improved photoefficiency, and a wider absorption band
than titania alone. Composites of TiO.sub.2/C have also been shown
to improve the energy and power density of electrochemical cells
and enhance the storage capacity and reversibility of
hydrogen-storage materials. The efficacy of these composite
materials depends mainly on the crystallinity and surface area of
the titania species. As a result, significant efforts have been
focused on the design of high surface area composites containing
either rutile or anatase TiO.sub.2. One approach to the fabrication
of these composites has been the incorporation of the titania
within high surface area supports or scaffolds. While this approach
has generated a variety of novel titania composites, the surface
areas of the composites are typically lower than those of the
scaffolds themselves. The decrease in surface area is generally
attributed to blocking of the micropores in the support by the
deposited titania, decreasing the accessible surface area. The
design of a high surface area support containing bimodal porosity
(macro- and micropores) could limit the detrimental effects
associated with pore-plugging, thereby providing a route to a new
class of high surface area titania composites.
[0079] Applicants recently reported the synthesis of activated
carbon aerogel (ACA) monoliths that exhibited hierarchical porosity
and surface areas in excess of 3000 m.sup.2 g.sup.-1. Applicants
use these materials as scaffolds for the synthesis of high surface
area titania and titanium carbonitride (TiCN) composites. The
composites are prepared through coating the inner surfaces of
monolithic ACA templates with a layer of sol-gel-derived titania,
yielding the TiO.sub.2/ACA composite. In a typical synthesis, ACA
parts were immersed in the TiO.sub.2 sol-gel solution and full
infiltration of the ACA pore network by the sol-gel solution was
achieved under vacuum. After drying, the amorphous TiO.sub.2
overcoat in the composite can then be converted to either anatase
TiO.sub.2 or titanium carbonitride through heat treatment under
different conditions. To convert the amorphous TiO.sub.2 layer to
anatase, the as-prepared. TiO.sub.2/ACA part was heated in air at
400.degree. C. for 2 hours. Alternatively, to prepare the
TiCN-coated ACA composite, the as-prepared TiO.sub.2/ACA part was
heated under flowing nitrogen at 1400.degree. C. for 4 hours. In
both cases, the heat-treated composite materials exhibit extremely
high BET surface areas (>1800 m.sup.2 g.sup.-1) and retain the
porous network structure of the monolithic ACA support. Because of
the technological importance of titania and its well-documented
conversion to TiC.sub.1-xN.sub.x (0<x<1) via carbothermal
reduction, these systems were chosen to demonstrate the potential
of the ACA as a scaffolding material. Nevertheless, the approach
described here is general and can be applied to the fabrication of
other high surface area metal oxide, metal nitride and metal
carbide composites of interest.
[0080] The microstructures of the titania-ACA composites were
evaluated using scanning electron microscopy FIGS. 6A-D and
transmission electron microscopy FIGS. 7A-C. SEM images of
as-prepared TiO.sub.2/ACA FIGS. 6C-D show the same trabecular
structure and texture as observed in the pristine ACA FIGS. 6A-B.
The presence of the TiO.sub.2 aerogel layer on the surface of the
ACA can be seen in images of the as-prepared TiO.sub.2/ACA
composites. Interestingly, the TiO.sub.2 aerogel appears to have
formed primarily on the surfaces of the ACA despite the fact that
the TiO.sub.2 sol-gel solution filled the entire pore volume of the
support. As seen in FIGS. 7C-D and FIG. 7A, the open pore volume in
the ACA composite is only sparsely populated with TiO.sub.2
particles. This observation indicates that nucleation of the
TiO.sub.2 particles during the sol-gel reaction preferentially
occurs at the surface of the ACA. After heat treatment at
400.degree. C., the texture of the TiO2/ACA composite appears to
roughen, apparently due to the formation of anatase TiO2
nanocrystals on the ACA surface FIG. 6E-F and FIG. 7C. Further
changes in texture are seen after carbothermal reduction of the
surface layer of TiO2 to TiCN FIG. 6G-H and FIG. 7C. In the
TiCN/ACA composite, cubic TiCN crystals ranging in size from 10 to
100 nm are clearly visible on the ACA surface. The continuous
nature of the crystalline TiCN layer suggests that the deposited
TiO2 completely coated the entire surface of the ACA support. With
the bulk of the TiO2 deposited at the ACA surface, the number of
TiO2 particles formed in sol filling the free space in the ACA is
greatly reduced.
[0081] Thermal gravimetric analysis in air was used to determine
the TiO2 content in the as-prepared and annealed TiO2/ACA
composites as well as the TiCN content in the TiCN/ACA composite.
As expected, combustion of the pristine ACA begins oxidizing at
400.degree. C. and the material is completely consumed by
600.degree. C. The onset of mass loss for the annealed TiO2/ACA
composite is similar to that of the ACA, but the material retains
20% of its original mass due to the presence of the TiO2 overcoat
Table III. In contrast to the ACA and TiO2/ACA materials, the
TiCN/ACA composite exhibits a slight weight gain at about
350.degree. C. prior to combustion of the carbon support. The
increase in mass can be attributed to oxidation of the TiCN layer
(molecular weight of 60-62) to TiO2 (molecular weight of 80).
Interestingly, complete oxidation of the ACA support in the
TiCN/ACA composite does not occur until 680.degree. C. as compared
to 600.degree. C. for the other samples, suggesting that the TiCN
completely covers the ACA surface, providing an effective barrier
to oxygen diffusion. In addition, the energy dispersive X-ray
spectroscopy (EDX) element mapping of the TiCN/ACA shows an even
distribution of Ti, C, and N, consistent with a TiCN layer covering
most of the ACA, as observed in the SEM and TEM images. Only after
the TiCN is converted to the oxide does combustion of the ACA
occur. The remaining 18 wt % TiO2 from combustion of the TiCN/ACA
composite implies a starting TiCN content of 14 wt %.
TABLE-US-00003 TABLE III Physical properties for the ACA support,
TiO.sub.2 aerogels and the ACA composites Monolithic TiO.sub.2/
density/g S.sub.BET/ V.sub.total/ V.sub.micro/ Material wt % cm
m.sup.2 g cm.sup.3 g cm.sup.3 g ACA 0 0.140 2455 1.05 0.42
TiO.sub.2 aerogel 78 0.193 237 0.53 -- (as prepared) TiO.sub.2
aerogel 99 n.a..sup.a 141 0.33 -- (heat-treated) TiO.sub.2/ACA 15
0.230 1507 0.91 0.50 (as-prepared) TiO.sub.2/ACA 20 0.104 2054 1.30
0.61 (heat-treated) TiCN/ACA .sup. 14.sup.b 0.148 1838 1.01 0.43
.sup.aThe heat-treated TiO.sub.2 aerogel was isolated as a powder.
.sup.bTiCN content shown for TiCN/ACA. indicates data missing or
illegible when filed
[0082] Powder XRD was used to determine the crystalline phases of
the heat-treated TiO.sub.2/ACA and TiCN/ACA composites. For
comparison, the XRD pattern of the ACA was also included. The XRD
pattern for the as-prepared TiO.sub.2/ACA (no heat treatment) was
very similar to that of the ACA, likely due to the amorphous nature
of the titania, and is, therefore, not shown. The largest peaks in
the diffraction pattern for the ACA material can be attributed to
the (100) and (101) graphite peaks (PDF #41-1487). These peaks are
also visible in the diffraction patterns for the heat-treated
TiO.sub.2/ACA and TiCN/ACA composites due to the presence of the
ACA support. The remaining peaks in the XRD pattern for the
annealed TiO.sub.2/ACA composite can be indexed to the anatase
phase of TiO2 (PDF #21-1272). Analysis of the peaks using the
Scherrer equation indicates the average crystallite size is about 9
nm, in agreement with the small size of the crystals observed by
electron microscopy. The XRD peaks for the TiCN/ACA composite
indicate the presence of the osbornite crystalline phase of TiCN
(PDF #06-0642) on the ACA support. The calculated lattice
parameter, a, for the TiCN in the TiCN/ACA, 4.248 A, is in good
agreement with TiC.sub.1-xN.sub.x (x=0.90) and very close to the
value for pure TiN, 4.240. The high nitrogen content is consistent
with EDX results showing a Ti:N ratio of close to one. The average
crystallites size calculated from the XRD data (about 20 nm)
correlates with the size range of the cubic crystals observed in
SEM and TEM analysis. Therefore, based on the XRD data, the
heat-treated TiO.sub.2/ACA composite contained purely anatase
nanocrystals, and full reduction of TiO.sub.2 to TiCN was achieved
in the TiCN/ACA composite to create a highly nitrogen-enriched
layer of TiCN nanocrystals on the ACA surface.
[0083] The textural properties of the TiO.sub.2/ACA and TiCN/ACA
composites were evaluated using nitrogen adsorption/desorption
analysis Table III. For comparison, data for the ACA and TiO.sub.2
aerogel (before and after heat treatment) are also included in
Table III. Nitrogen adsorption/desorption plots for the ACA and the
composites. Each of the composites exhibited type II nitrogen
isotherms, indicating a mostly macroporous (<2 nm) material with
the remaining pore volume primarily in the large meso- and
macropore (>90 nm) range. Coating of the ACA framework with TiO2
clearly results in a significant decrease in BET surface area (1507
m.sup.2 g.sup.-1) relative to the uncoated ACA. Nevertheless, the
surface area of the as-prepared TiO.sub.2/ACA composite represents
almost an order of magnitude improvement over that of the
as-prepared TiO2 aerogel. Retention of such a large BET surface
area in the coated material suggests that the ACA is less
susceptible to the negative effects of pore-plugging observed in
other scaffold materials, such as activated carbons. Additionally,
heat treatment of the as-prepared TiO.sub.2/ACA leads to a 36%
increase in surface area in the annealed composite (2054 m.sup.2
g.sup.-1). This observation is in contrast to the sharp decrease in
surface area that occurs upon annealing of the bulk TiO.sub.2
aerogel prepared without the scaffold. The increased surface area
and pore volume in the annealed composite indicate that the ACA
support prevents coarsening and collapse of the TiO.sub.2 coating
during heat treatment, even as the amorphous titania is converted
to the anatase phase. The presence of high-surface area SiO.sub.2
has been shown to have similar effects on the temperature stability
of pores in TiO.sub.2 gels. In addition, correspondingly lower
density of the annealed TiO2/ACA (relative to as-prepared
TiO.sub.2/ACA) is consistent with a lack of pore collapse and
likely contributes to the observed textural properties. Similarly,
the TiCN/ACA composite also exhibits increased surface area and
pore volume relative to the as-prepared TiO.sub.2/ACA composite
Table III. The increased surface area can be attributed to the
additional porosity created by the removal of carbon from the ACA
support (in the form of gaseous CO) that occurs during carbothermal
reduction. Similar increases in surface area have been reported
under similar conditions during the conversion of TiO.sub.2
(rutile) and carbon (furnace black or graphite) mixtures to TiCN.
While the surface area and pore volume for the TiCN/ACA composite
are slightly lower than those of the heat-treated TiO.sub.2/ACA,
the textural properties are still quite close to those of the
original ACA. This observation demonstrates the flexibility of the
ACA scaffold for creating a variety of high surface area oxide,
carbide and nitride materials.
[0084] In a typical synthesis, titanium(IV) ethoxide (1 g, 0.0125
mol) and ethanol (3.57 g, 0.0776 mol), hydrochloric acid (71.4
.mu.L), and water (851 .mu.L) were mixed in an ice bath, followed
by the addition of propylene oxide (0.357 g, 0.00616 mol) to
prepare the titania sol. An activated carbon aerogel monolith was
immersed in the titania sol in a glass vial and held under vacuum
to ensure full penetration of the sol in the carbon aerogel. The
reaction mixture was then cured at room temperature for 24 h. The
wet composite was washed in ethanol and dried by supercritical
extraction in CO2 to yield the TiO.sub.2/ACA composite Annealing
the as-prepared TiO.sub.2/ACA composite in air at 400.degree. C.
for 2 h was required to convert the amorphous titania layer on the
ACA to the anatase phase. Alternatively, heating the as-prepared
TiO.sub.2/ACA composite in flowing nitrogen at 1400.degree. C. for
4 h produced the TiCN/ACA composite.
[0085] Powder X-ray diffraction (XRD) analysis of the samples was
performed with Cu Ka radiation on a Scintag PAD-V X-ray
diffractometer. TiO.sub.2 (anatase) powder was used as a standard.
Bulk densities of the monoliths were determined from the physical
dimensions and mass of each sample. Scanning electron microscopy
(SEM) and energy-dispersive X-ray spectroscopy (EDX)
characterization was performed on a JEOL 7401-F at 5-10 keV (20 mA)
in SEI mode with a working distance of 2-8 mm. Transmission
electron microscopy (TEM) characterization was performed on a JEOL
JEM-200CX electron microscope operated at 200 kV. Thermogravimetric
analysis (TGA) was performed on a Shimadzu TGA 50 thermogravimetric
analyzer to determine TiO2 and TiCN contents. Samples were heated
in flowing air at 10 sccm to 1000.degree. C. at 10.degree. C.
min.sup.-1 in alumina boats. The weight fraction of material
remaining was assumed to be pure stoichiometric TiO.sub.2. The TiCN
content of the TiCN/ACA was calculated from the weight fraction of
TiO2 remaining after heating to 1000.degree. C. in air assuming
full oxidation of initial TiCN content. Energy dispersive
spectroscopy confirmed that only TiO.sub.2 remained after TGA was
performed. Surface area determination and pore volume analysis were
performed by Brunauer-Emmett-Teller (BET) and
Barrett-Joyner-Halenda (BJH) methods using an ASAP 2000 surface
area analyzer (Micromeritics Instrument Corporation). Samples of
approximately 0.1 g were heated to 300.degree. C. under vacuum
(10.sup.-5 Torr) for at least 24 h to remove all adsorbed species,
prior to analysis.
[0086] The synthesis and characterization of TiO.sub.2/ACA and
TiCN/ACA composites with the highest surfaces areas yet reported
has been described. The flexibility of the described method should
allow for synthesis of other high surface area metal oxides,
carbides, and nitrides through the use of supports with bimodal
porosity, like the ACA, to minimize pore-plugging effects. This new
class of high-surface area materials should be especially
advantageous in technologies such as catalysis and energy storage
where high surface area and accessible pore volume are desired.
[0087] This high surface area TiO.sub.2/C and TiCN/C composites
example is described in greater detail in the journal article "high
surface area TiO.sub.2/C and TiCN/C composites," by Marcus A.
Worsley, Joshua D. Kuntz, Octavio Cervantes, T. Yong-Jin Han, Alex
E. Gash, Joe H. Satcher, Jr and Theodore F. Baumann, Journal of
Materials Chemistry, 2009, 19, 7146-7150, which is incorporated
herein in its entirety by this reference for all purposes.
SiO.sub.2/ACA and SiC/ACA composites
[0088] Other embodiments described herein provide a silicon
oxide-carbon composite that includes a carbon aerogel with an oxide
overcoat, for example a silicon oxide overcoat. Other embodiments
described herein provide a silicon carbide-carbon composite that
includes a carbon aerogel with a carbide overcoat, for example a
silicon carbide overcoat. The silicon carbide-carbon composite is
made by providing a carbon aerogel, immersing the carbon aerogel in
a silicon oxide sol under a vacuum, returning the carbon aerogel
with the silicon oxide sol to atmospheric pressure, curing the
carbon aerogel with the silicon oxide sol-gel at room temperature
to produce a silicon oxide-carbon wet gel composite, and drying the
silicon oxide-carbon wet gel composite to produce the silicon
oxide-carbon aerogel composite. The silicon oxide-carbon aerogel
composite can then be heated, for example at a temperature of
1500.degree. C. in argon to reduce the silicon oxide to silicon
carbide.
[0089] The synthesis and characterization of high surface area
carbon-supported silica and silicon carbide aerogels are
described.
[0090] An activated carbon aerogel with surface area greater than
3000 m.sup.2/g was used to as a support for the sol-gel deposition
of silica. The resulting silica-coated carbon aerogel retained a
surface area greater than 2000 m.sup.2/g and showed improved
thermal stability in air. The carbon-supported silicon carbide
aerogel was made by the carbothermal reduction of the silica-coated
carbon aerogel under flowing Ar at 1500.degree. C. The resulting
monolith maintained a surface area greater than 2000 m.sup.2/g and
was stable to temperatures approaching 600.degree. C. over
100.degree. C. higher than that of the pristine carbon aerogel.
[0091] The microstructures of the SiO.sub.2/ACA and SiC/ACA
composites were evaluated using scanning electron microscopy as
shown in FIGS. 8A-D. SEM images of as-prepared SiO.sub.2/ACA (FIG.
8A-B) show the same trabecular structure and texture as observed in
pristine ACA. The presence of the SiO.sub.2 aerogel layer on the
surface of the ACA can be seen in images of the as-prepared
SiO.sub.2/ACA composites. Interestingly, the SiO.sub.2 aerogel
appears to have formed primarily on the surfaces of the ACA despite
the fact that the SiO.sub.2 sol-gel solution filled the entire pore
volume of the support. As seen in FIGS. 8A-B, the open pore volume
in the ACA composite is only sparsely populated with SiO.sub.2
particles. This observation indicates that nucleation of the
SiO.sub.2 particles during the sol-gel reaction described above
preferentially occurs at the surface of the ACA. Further changes in
texture are seen after carbothermal reduction of the surface layer
of the SiO.sub.2 to SiC (FIGS. 8C and 8D). In the SiC/ACA
composite, virtually no particles are visible in the open pore
volume. In fact, the SiC/ACA appears to have the same texture as
the pristine ACA, suggesting the SiC forms a fairly conformal layer
on the ACA.
[0092] Energy dispersive x-ray analysis was used to track the
composition change of the composite during the carbothermal
reduction. Oxygen atomic content was used to determine the level of
reduction as the SiO.sub.2/ACA was populated with SiO.sub.2
particles. This observation indicates that nucleation of the
SiO.sub.2 particles during the sol-gel reaction preferentially
occurs at the surface of the ACA. Further changes in texture are
seen after carbothermal reduction of the surface layer of SiO.sub.2
to SiC (FIG. 8A-D). In the SiC/ACA composite, virtually no
particles are visible in the open pore volume. The SiC/ACA
composite comprises virtually no particles in the open pore volume,
appearing to have the same texture as pristine ACA. Similar results
were observed in the case of TiO.sub.2 and TiCN on ACA as described
above.
[0093] The carbothermal reduction was considered complete when the
0 content in the solid phase is reduced to zero. At 1500.degree. C.
the 0 content drops from 12% to 3% within the first 10 minutes
suggesting formation of an Si.sub.xO.sub.yC phase. The Si and C
content show corresponding increases during this intial period. The
0 content then slowly decreases to zero over the next 5 h. The Si
and C content remain fairly constant. Based on these results, it
was concluded that a 5 h treatment at 1500.degree. C. was
sufficient to completely convert the SiO.sub.2 layer in the
SiO2/ACA to SiC. This is consistent with literature on SiC
synthesis.
[0094] Powder XRD was used to confirm the presence of SiC in the
SiC/ACA composite. For comparison, the XRD pattern of the
as-prepared SiO.sub.2/ACA was also included. The XRD pattern for
the pristine ACA is identical to that of the SiO.sub.2/ACA, due to
the amorphous nature of the as-prepared silica, and is, therefore,
not shown. The largest peaks in the diffraction pattern for the
SiO.sub.2/ACA material can be attributed to the (100) and (101)
graphite peaks. These peaks are also visible in the diffraction
pattern for the SiC/ACA composites due to the presence of the ACA
support. The remaining peaks in the XRD pattern for the SiC/ACA
composite can be indexed to moissanite SiC. Analysis of the peaks
using the Scherrer equation indicates the average crystallite size
is .about.26 nm. Therefore, based on the XRD and EDX data, full
reduction of SiO.sub.2 to SiC was achieved in the SiC/ACA composite
to create a layer of SiC nanocrystals on the ACA surface.
[0095] Thermal gravimetric analysis in air was used to determine
the thermal stability of the SiO.sub.2/ACA and SiC/ACA, as well as
the SiO.sub.2 and SiC content. As expected, combustion of the
pristine ACA begins at 400.degree. C. and the material is
completely consumed by 600.degree. C. The mass loss event below
200.degree. C. for the SiO.sub.2/ACA is due to organic impurities
from the as-prepared SiO.sub.2. The onset of ACA mass loss for the
SiO.sub.2/ACA composite is .about.100.degree. C. higher than that
of the pristine ACA, suggesting that the SiO.sub.2 covers the ACA
surface fairly well and forms a decent barrier to oxygen diffusion.
Similar improvements in thermal stability were noted with a
TiCN/ACA. In the case of TiCN/ACA, the TiCN was completely oxidized
to TiO.sub.2 in the process, in contrast to the SiO.sub.2 in the
SiO.sub.2/ACA. For the SiO.sub.2/ACA, complete oxidation of the ACA
occurs at 690.degree. C. This material retains 15% of its original
mass due to the presence of the SiO.sub.2 overcoat. Further
improvements in thermal stability are observed in the SiC/ACA
composite. Mass loss does not begin until close to 600.degree. C.
and complete oxidation of the carbon support does not occur until
720.degree. C. Like the SiO.sub.2/ACA, this improved thermal
stability suggests that the SiC completely covers the ACA surface,
providing an effective barrier to oxygen diffusion. The remaining
10% material remaining represents oxidation-resistant SiC.
[0096] The textural properties of the SiO.sub.2/ACA and SiC/ACA
composites were evaluated using nitrogen adsorption/desorption
analysis (Table IV). Each of the composites exhibited type II
nitrogen isotherms, indicating a mostly microporous <2 nm)
material with the remaining pore volume primarily in the large
meso- and macropore (>90 nm) range. Coating of the ACA framework
with SiO.sub.2 clearly results in a significant decrease in BET
surface area (2288 m.sup.2/g) relative to the uncoated ACA.
Nevertheless, the surface area of the as-prepared SiO.sub.2/ACA
composite represents almost an order of magnitude improvement over
that of the as-prepared SiO.sub.2 aerogel. Retention of such a
large BET surface area in the coated material suggests that the ACA
is less susceptible to the negative effects of pore-plugging
observed in other scaffold materials, such as activated
carbons.
TABLE-US-00004 TABLE IV Monolithic SiO.sub.2 Density S.sub.BET,
V.sub.total V.sub.micro, Material wt % g/cm.sup.3 m.sup.2/g
cm.sup.3/g cm.sup.3/g ACA 0 0.083 3289 1.61 0.85 SiO.sub.2 aerogel
91 0.120 350 0.98 -- SiO.sub.2/ACA 15 0.147 2288 1.34 0.80 SiC/ACA
.sup. 10.sup.a 0.087 2156 1.15 0.78 .sup.aSiC content shown for
SiC/ACA
[0097] After carbothermal reduction, the textural properties show
little change. There is small loss of surface area and pore volume,
likely due to sintering that occurs during the reduction process.
While the surface area and pore volume for the SiC/ACA composite
are slightly lower than those of the heat-treated SiO.sub.2/ACA,
the textural properties are still quite close to those of the
original ACA. This observation demonstrates the effectiveness of
the ACA scaffold for creating high surface area oxide and carbide
materials.
[0098] In a typical synthesis, trimethoxysilane (IV) ethoxide (4.1
g) and methanol (14 g), ammonium hydroxide (200 ml), and water (1.5
g) were mixed to prepare a silica sol. An activated carbon aerogel
(ACA) monolith with surface area greater than 3000 m.sup.2/g was
immersed in the silica sol in a glass vial and held under vacuum to
ensure full penetration of the sol in the carbon aerogel. The
reaction mixture was then cured at room temperature for 24 h. The
wet composite was washed in ethanol and dried by supercritical
extraction in CO.sub.2 to yield an SiO.sub.2/ACA composite. Heating
the as-prepared SiO.sub.2/ACA composite in flowing argon at
1500.degree. C. for about five hours produced an SiC/ACA composite.
While not bound to a particular mechanism, it is believed that the
carbon-supported silicon carbide aerogel was made by the
carbothermal reduction of the silica-coated carbon aerogel.
[0099] Powder x-ray diffraction (XRD) analysis of the samples was
performed with Cu K, radiation on a Scintag PAD-V X-ray
diffractometer. TiO.sub.2(anatase) powder was used as a standard.
Bulk densities of the monoliths were determined from the physical
dimensions and mass of each sample. Scanning electron microscopy
(SEM) and energy-dispersive x-ray spectroscopy (EDX)
characterization was performed on a JEOL 7401-F at 5-10 keV (20mA)
in SEI mode with a working distance of 2-8 mm. Transmission
electron microscopy (TEM) characterization was performed on a JEOL
JEM-200CX Electron Microscope operated at 200 kV. Thermogravimetric
analysis (TGA) was performed on a Shimadzu TGA 50 Thermogravimetric
Analyzer to determine SiO.sub.2 and SiC content. Samples were
heated in flowing air at 10 sccm to 1000.degree. C. at 10.degree.
C./min in alumina boats. The weight fraction of material remaining
was assumed to be pure stoichiometric SiO.sub.2 and SiC. Surface
area determination and pore volume analysis were performed by
Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH)
methods using an ASAP 2000 Surface Area Analyzer (Micromeritics
Instrument Corporation).42 Samples of approximately 0.1 g were
heated to 300.degree. C. under vacuum (10.sup.-5 Torr) for at least
24 h to remove all adsorbed species, prior to analysis.
[0100] The synthesis and characterization of SiO.sub.2/ACA and
SiC/ACA composites with the highest surfaces areas yet reported has
been described. The flexibility of the described method should
allow for synthesis of other high surface area metal oxides,
carbides, and nitrides through the use of supports with bimodal
porosity, like the ACA, to minimize pore-plugging effects. This new
class of high-surface area materials should be especially
advantageous in technologies such as catalysis and energy storage
where high surface area and accessible pore volume are desired.
[0101] Referring now to FIG. 9 a flow chart illustrates one
embodiment of a method of making a carbon aerogel oxide composite
in accordance with the present invention. The method is designated
generally by the reference number 900. The method 900 includes a
number of steps. The steps include dispersing nanotubes in an
aqueous media or other media to form a suspension, adding reactants
and catalyst to the suspension to create a reaction mixture, curing
the reaction mixture to form a wet gel, drying the wet gel to
produce a dry gel, pyrolyzing the dry gel to produce a carbon
nanotube-based aerogel, immerse the carbon nanotube-based aerogel
in a metal oxide sol under a vacuum, returning the carbon
nanotube-based aerogel and the metal oxide sol to atmospheric
pressure, curing the metal oxide-carbon nanotube-based composite at
room temperature, and drying the metal oxide-carbon nanotube-based
wet gel composite producing an metal oxide-carbon composite. In one
embodiment the step of immersing the carbon nanotube-based aerogel
in a metal oxide sol under a vacuum comprises immersing the carbon
nanotube-based aerogel in titanium dioxide. In one embodiment the
step of immersing the carbon nanotube-based aerogel in a metal
oxide sol under a vacuum comprises immersing the carbon
nanotube-based aerogel in a metal oxide sol made from Mn, Fe, Co,
Ni, Cu, Sn, Al, Si, Zn, Zr sol-gel precursors in combination with
catalyst, and sol-gel forming components. Referring again to FIG.
9, the method 900 includes a number of steps. The steps shown
include the steps described below.
[0102] Step number 901 is "Obtain resorcinol, formaldehyde, sodium
carbonate, sodium dodecylbenzene sulfonate (SDBS) and purified
double-walled nanotubes (DWNT)."
[0103] Step number 902 is "Purified DWNTS suspended in aqueous
solution containing SDBS."
[0104] Step number 903 is "Dispersal of DWNTS in aqueous surfactant
solution containing SDBS using sonication."
[0105] Step number 904 is "Resorcinol, formaldehyde and sodium
carbonate catalyst added to the reaction solution."
[0106] Step number 905 is "Sol-Gel mixture transferred to glass
molds sealed and cured in oven at 85.degree. C. for 72 hours."
[0107] Step number 906 is "Resulting gel removed from mold and
washed with acetone for 72 hours to remove all water from pores of
gel network."
[0108] Step number 907 is "Wet gel dried with supercritical
CO.sub.2 and pyrolyzed at 1050.degree. C. under N.sub.2 atmosphere
for 3 hours."
[0109] Step number 908 is "Resulting composite material (CA-DWNT)
isolated as black cylinder monoliths."
[0110] Step number 909 is "Immerse in titanium dioxide (TiO.sub.2)
sol: infiltration of pore network achieved under vacuum."
[0111] Step number 910 is "Return to atmospheric pressure and dry
wet composite using supercritical CO.sub.2 producing a metal
oxide-carbon composite.
[0112] Referring now to FIG. 10 a flow chart illustrates an
embodiment of a method of making a metal oxide-carbon aerogel
composite in accordance with the present invention. The method is
designated generally by the reference number 1000. The method 1000
includes a number of steps. The steps include providing an aqueous
media or other media to form a suspension, adding reactants and
catalyst to the suspension to create a reaction mixture, curing the
reaction mixture to form a wet gel, drying the wet gel to produce a
dry gel, pyrolyzing the dry gel to produce an aerogel, immerse the
aerogel in a metal oxide sol under a vacuum, returning the aerogel
and the metal oxide sol to atmospheric pressure, curing the metal
oxide sol-infiltrated carbon aerogel, and drying the metal
oxide-carbon wet gel composite producing a metal oxide-carbon
aerogel composite. In one embodiment the step of immersing the
carbon aerogel in a metal oxide sol under a vacuum comprises
immersing the carbon aerogel in titanium dioxide sol. In one
embodiment the step of immersing the carbon aerogel in a metal
oxide sol under a vacuum comprises immersing the carbon aerogel in
a metal oxide sol made from Mn, Fe, Co, Ni, Cu, Zn, Zr sol-gel
precursors in combination with a catalyst, and sol-gel forming
components.
[0113] Referring again to FIG. 10, the method 1000 includes a
number of steps. The steps shown include the steps described
below.
[0114] Step number 1001 is "Resorcinol and 37% formaldehyde
solution dissolved in water."
[0115] Step number 1002 is "Add glacial acetic acid."
[0116] Step number 1003 is "Transferred to glass molds and cured at
80.degree. C. for 72 hours."
[0117] Step number 1004 is "Resultant organic hydrogels washed with
acetone to remove water and dried with supercritical CO.sub.2."
[0118] Step number 1005 is "Organic aerogels carbonized at
1050.degree. C. for 3 hours under N.sub.2 atmosphere."
[0119] Step number 1006 is "Carbon monoliths."
[0120] Step number 1007 is "Activating carbon aerogel by exposing
to stream of CO.sub.2 at 950.degree. for different soak times."
[0121] Step number 1008 is "Shorter activation time new porosity is
in the form of micropores."
[0122] Step number 1009 is "Longer activation time. The micropore
are widened to sizes that cross the micropore mesopore
boundry."
[0123] While the invention may be susceptible to various
modifications and alternative forms, specific embodiments have been
shown by way of example in the drawings and have been described in
detail herein. However, it should be understood that the invention
is not intended to be limited to the particular forms disclosed.
Rather, the invention is to cover all modifications, equivalents,
and alternatives falling within the spirit and scope of the
invention as defined by the following appended claims.
Additional embodiments
[0124] Additional embodiments are described in Worsley et al., J.
Mater. Chem., 2010, 20, 4840-4844, DOI: 10.1039/C0JM00661K.
[0125] Recently reported is the synthesis of activated carbon
aerogel (ACA) monoliths that exhibited hierarchical porosity and
surface areas in excess of 3000 m.sup.2g.sup.-1. (Reference 10)
Previously, it was demonstrated the potential of the ACA material
as a support by creating high surface area TiO.sub.2-coated and
TiCN-coated ACA composites. (Reference 11) Although both the
TiO.sub.2/ACA and TiCN/ACA maintained high surface areas, oxidation
of the composites occurred just above 400.degree. C. in air.
[0126] Herein is described the use the ACA material as scaffolds
for the synthesis of high surface area silica (SiO.sub.2) and
silicon carbide (SiC) composites with enhanced thermal stability.
Both SiO2 and SiC are known for their stability at high
temperatures. (References 6, 12-16) In addition to the advantages
such a SiO.sub.2/ACA and SiC/ACA material would have as catalyst
supports, these materials can have application in lithium ion
batteries and methane storage. (References 1, 2, 17)
[0127] The composites can be prepared through coating the inner
surfaces of monolithic ACA templates with a layer of sol-gel
derived silica, (referencel8) yielding the SiO.sub.2/ACA composite.
In a typical synthesis, ACA parts were immersed in the SiO.sub.2
sol-gel solution and full infiltration of the ACA pore network by
the sol-gel solution was achieved under vacuum. After drying, the
amorphous SiO.sub.2 overcoat in the composite can then be converted
to SiC by carbothermal reduction. To prepare the SiC-coated ACA
composite, the as-prepared SiO.sub.2/ACA part was heated under
flowing nitrogen at 1,500.degree. C. for 5 hours. The SiO.sub.2/ACA
and SiC/ACA exhibit extremely high BET surface areas (>2000
m.sup.2g.sup.-1) and retain the porous network structure of the
monolithic ACA support.
[0128] The surface area of the SiC-coated carbon aerogel can be,
for example, at least 60%, or at least 70%, or at least 80%, or at
least 90% of the surface area of the original carbon aerogel before
the SiC is introduced.
[0129] The BET surface area can be, for example, greater than 2,100
m.sup.2g.sup.-1 or greater than 2,200 m.sup.2g.sup.-1.
[0130] The aerogel can provide a bimodal porosity. For example, one
mode for porosity can have a size of about, for example, 10 nm or
less, or about 2 nm or less, whereas the other mode for porosity
can have a larger pore size of, for example, about 100 nm or more,
or about 1 micron or more.
[0131] The sol solution can be adapted so that the rate of reaction
can be controlled. For example, the rate can be slowed by reducing
the concentration of the catalyst. One skilled in the art can vary
the concentration of the sol gel catalyst to determine a best
concentration or ranges of concentrations for the catalyst. This
factor can help reduce pore plugging. For example, it can help
reduce pore plugging to have the sol particles nucleate on the
surface of the pores which keeps the center of the pores relatively
clear. If, for example, sol particles preferred to nucleate in
solution (in the middle of the pore away from the surface) it could
more easily grow to block the pore. One can have macropores to
allow access to the smaller pores without plugging.
[0132] The reduction of silica to silicon carbide occurs according
to the following equation (see references 19, 20):
SiO.sub.2+3C(s).fwdarw.SiC(s)+2CO(g)
[0133] Surface areas determination and pore volume analysis is
described in reference 21. Additional methods for making the
coatings include atomic layer deposition (ALD). In additional
embodiments, carbon aerogels including activated carbon aerogels
can also be coated with alumina. The alumina coating can be
deposited by sol gel or atomic layer deposition (ALD) methods. In
other embodiments, the alumina is coated on the aerogel by
immersing the aerogel in an alumina sol in a similar method as
described above for the silicon oxide coating.
[0134] For example, in a typical synthesis, an activated carbon
aerogel (ACA) monolith with surface area greater than 3,000
m.sup.2/g is placed under vacuum and heated to >150.degree. C.
Atomic layer deposition of alumina on the inner surfaces of the
carbon aerogel can be achieved by sequentially pulsing
Al(CH.sub.3).sub.3 and H.sub.2O into the vacuum chamber with
sufficient purge steps in between. When the desired thickness of
alumina is achieved, the Al.sub.2O.sub.3/ACA composite can be
removed from the chamber.
[0135] In a typical synthesis, for example, aluminum salts, such as
aluminum chloride, in either water and/or alcohol, are combined
with an organic epoxide (e.g. ethylene oxide, propylene oxide) to
prepare an alumina sol. An activated carbon aerogel (ACA) monolith
with surface area greater than 3000 m.sup.2/g can be immersed in
the alumina sol in a glass vial and held under vacuum to ensure
full penetration of the sol in the carbon aerogel. The reaction
mixture can be then cured at room temperature for 24 h. The wet
composite can be washed in ethanol and dried by supercritical
extraction in CO.sub.2 to yield an Al.sub.2O.sub.3/ACA
composite.
REFERENCES
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[0156] 21 Gregg et al., Adsorption, Surface Area and Porosity,
Academic, London, 2nd edn, 1982. 0 [0157] 14 ADDITIONAL EMBODIMENTS
DESCRIBED IN U,S, Ser. No. 12/694,425
[0158] Fourteen embodiments were described and claimed in U.S. Ser.
No. 12/694,425 as described below.
[0159] Embodiment 1. A method of making a metal oxide-carbon
composite, comprising the steps of: providing a carbon aerogel,
immersing said carbon aerogel in a metal oxide sol under a vacuum,
returning said carbon aerogel with said metal oxide sol to
atmospheric pressure, curing said carbon aerogel with said metal
oxide sol to produce a metal oxide-carbon wet gel composite, and
drying said metal oxide-carbon wet gel composite so producing a
metal oxide-carbon composite.
[0160] Embodiment 2. The method of making a metal oxide-carbon
composite of embodiment 1 wherein said step of providing a carbon
aerogel comprises providing an activated carbon aerogel.
[0161] Embodiment 3. The method of making a metal oxide-carbon
composite of embodiment 1 wherein said step of providing a carbon
aerogel comprises providing a carbon aerogel carbon aerogel with
carbon nanotubes that make said carbon aerogel mechanically
robust.
[0162] Embodiment 4. The method of making a metal oxide-carbon
composite of embodiment 1 wherein said step of immersing said
carbon aerogel in a metal oxide sol under a vacuum comprises
immersing said carbon aerogel in titanium dioxide.
[0163] Embodiment 5. The method of making a metal oxide-carbon
composite of embodiment 1 wherein said step of immersing said
carbon aerogel in a metal oxide sol under a vacuum comprises
immersing said carbon aerogel in a metal oxide sol made from Mn,
Fe, Co, Ni, Cu, Zn, Zr salts in combination with propylene oxide,
and sol-gel forming components.
[0164] Embodiment 6. The method of making a metal oxide-carbon
composite of embodiment 1 wherein said step of immersing said
carbon aerogel in a metal oxide sol under a vacuum comprises
immersing said carbon aerogel in a metal oxide sol for metal
species including but not limited to manganese, iron, cobalt,
nickel, copper, zinc, zirconium, tin, aluminum and chromium.
[0165] Embodiment 7. A method of making a metal oxide-carbon
composite, comprising the steps of: providing an aqueous media or
other media to form a suspension, adding reactants and catalyst to
said suspension to create a reaction mixture, curing said reaction
mixture to form a wet gel, drying said wet gel to produce a dry
gel, pyrolyzing said dry gel to produce an aerogel, immerse said
aerogel in a metal oxide sol under a vacuum, returning said aerogel
and said metal oxide sol to atmospheric pressure, curing said sol,
and drying said sol-gel producing a metal oxide-carbon
composite.
[0166] Embodiment 8. A metal oxide-carbon composite, comprising: a
carbon aerogel, said carbon aerogel having inner surfaces, andan
oxide coating said inner surfaces of said carbon aerogel providing
a metal oxide-carbon composite.
[0167] Embodiment 9. The metal oxide-carbon composite of claim 8
wherein said carbon aerogel is a carbon aerogel with carbon
nanotubes that make said carbon aerogel mechanically robust.
[0168] Embodiment 10. The metal oxide-carbon composite of
embodiment 8 wherein said carbon aerogel is an activated carbon
aerogel.
[0169] Embodiment 11. The metal oxide-carbon composite of
embodiment 8 wherein said oxide is titanium oxide.
[0170] Embodiment 12. The metal oxide-carbon composite of
embodiment 8 wherein said oxide is an oxide from metal oxide made
with forming precursors including but not limited to manganese or
iron or cobalt or nickel or copper or zinc or zirconium or aluminum
or silicon or tin salts or alkoxides.
[0171] Embodiment 13. A metal oxide-carbon composite, comprising: a
carbon aerogel with carbon nanotubes that make said carbon aerogel
mechanically robust, said carbon aerogel having inner surfaces,
andan oxide coating said inner surfaces of said carbon aerogel
providing an metal oxide-carbon composite.
[0172] Embodiment 14. A metal oxide-carbon composite, comprising:
an activated carbon aerogel, said activated carbon aerogel having
inner surfaces, and an oxide coating said inner surfaces of said
activated carbon aerogel providing an metal oxide-carbon
composite.
[0173] This concludes the 14 embodiments.
* * * * *